A cleaning company uses 10 lbs each of chemicals A, B and C for each house it cleans. After some quality complaints, the company has decided to increase its use of chemical A by an additional 10 lbs for each house. By what % has productivity (houses per pound of chemical) fallen?

A) 0%
B) 10%
C) 15%
D) 25%
E) 33%

Let's assume the company initially cleaned one house and used 10 lbs of each chemical, so the productivity was 1 house per pound of chemical.
Now, the company will use 20 lbs of chemical A for one house, which means it will use 10 lbs less of chemicals B and C.
The new productivity will be:
1 house / (20 lbs A + 10 lbs B + 10 lbs C) = 1 house / 40 lbs of chemicals
The old productivity was 1 house / (10 lbs A + 10 lbs B + 10 lbs C) = 1 house / 30 lbs of chemicals.
The percentage change in productivity is:
[(new productivity - old productivity) / old productivity] x 100%
= [(1/40 - 1/30) / (1/30)] x 100%
= 25%
So, the productivity has fallen by 25%. Therefore, the answer is D.